An X-ray generator is capable of reducing invalid exposure caused due to wave tails, which do not contribute to the quality of an X-ray image, by preventing the wave tails from being supplied to an X-ray tube. An X-ray imaging apparatus includes the X-ray generator and a method of reducing the invalid exposure is implemented using the X-ray generator. The X-ray generator includes an X-ray tube to generate X-rays, and a high-voltage generator to supply a voltage having a pulse waveform to the X-ray tube. The high-voltage generator includes a high-voltage tank to generate a high voltage, a switch connected to an output terminal of the high-voltage tank and turned on or off, and a resistor located outside the high-voltage tank to receive a wave tail voltage having the pulse waveform and to consume power generated due to wave tails.